Welding apparatus and welding system



Sept. 22, 1953 l: J. PICKERING .WELDING APPARATUS AND WELDING SYSTEM Filed: 001; 5 1951 4 Sheets-Sheet 1 FIG. I

INVENTOR q T-roRNz Sept. 22, 1953 l. J. PICKERING 2,653,277

WELDING APPARATUS AND WELDING SYSTEM Filed Oct. 5; 1951 4 Sheets-Sheet 2 INVEN TOR Sept. 22, 1953 I. J. PICKERING WELDING APPARATUS AND WELDING SYSTEM 4 Sheets-Sheet 3 Filed Oct. 5, 1951 I nun FIG.4

INVENTo- A JACK Pomp/'4 YM P 22, 1953 l. J. PlCKERlNG 2,653,277

WELDING APPARATUS AND WELDING SYSTEM Filed Oct. 5, 1951 4 Sheets-Sheet 4 INVENTOR Patented Sept. 22, 1953 UNITED STATES PATENT OFFICE wsmmc APPARATUS asp SYSTEM WELDING Ian Jack Pickering, Birmingham, England, assignor to The General Electric Company Limlted, London, England Application October 5, 1951, Serial No. 249,902

. In Grent Britain April 26, 1950 10 Claims. (01. 315 139) The present invention relates to welding 8-D- paratus and to welding systems employing a two-phase welding current supply and is applicable, for example, to such systems in which the two-phase welding current supply is obtained vention control apparatus for a two-phase arc welding system in which two welding electrodes are arranged to be used simultaneously and in close proximity, said electrodes and the work from a Scott-connected transformer set, though another electrically. The mechanical bonding of the two electrodes together can be effected, for example, but the use of the flux composition which normally surrounds the welding electrodes. It has been found with such arrangements that when the welding is stopped by removing the electrodes from the welding position in relation to the work, an arc persists between the two electrodes. It has been proposed to extinguish this are by providing a switch for cutting off the welding current. If this switch is arranged in the electrode holder there is a tendency for the holder to become unduly bulky and heavy so that manual welding is rendered more diflicult. As an alternative a foot operated switch may be arranged in the main cables between the welding transformer and the electrode holder. A further alternative consists in providing a contactor operated switch arranged either in the primary or the secondary winding of the welding transformer, the operator being provided with means for operating the contactor when desired. All these arrangements suffer from a disadvantage in that it is necessary for the-operator to perform an operation to switch off the welding current when he wishes to stop welding.

According to one aspect of the present invention apparatus for two-phase arc welding in which two welding electrodes are to be used simultaneously and in close proximity, said electrodes and the work to be welded being arranged to be connected to two phases and the neutral or common return line of a two-phase welding supply, is provided with automatic means for reducing the voltage between the electrodes to a value at which the inter-electrode arc is extinguished when the current or voltage conditions which obtain during welding operations are altered as. for instance, when the arcs between the electrodes and the work are extinguished.

According to another aspect of the present inbeing connected to two phases and the neutral or common return line of a two-phase welding supply, comprises automatic means for reducing the voltage between the electrodes to a value such that the inter-electrode arc is extinguished when the current or voltage conditions which obtain during welding operations are altered as, for instance, when the welding arcs between the electrodes and the work are extinguished.

In one arrangement in accordance with the present invention, the automatic means for reducing the voltage between the electrodes is responsive to the current flowing in'the neutral or common return line of the welding current .supply. The means responsive to the current flowing in the neutral line may include a saturable current transformer having a winding connected in the neutral line and arranged to feed the operating winding of a relay or contactor for effecting switching for interrupting any are which may persist between the two electrodes. The switching operation may be effected in the welding circuit or alternatively, if desired, it may be effected in the primary winding of a transformer having a secondary winding which is arranged to provide the welding current supply.

In addition to the means responsive to the current in the neutral or common return line of the welding supply, further means responsive to the decrease in the voltage between one or each of the welding lines and the common return line may be arranged to perform a switching operation to re-establish the normal welding conditions when one or both of the electrodes is brought into contact with the work so as to complete the welding circuit or circuits. This operation may be efiected by means of a relay responsive to the voltage between one of the welding lines and the common earth return line, this relay being arranged when the voltage drops to a low value to efiect a switching operation to reestablish the normal welding conditions.

As an alternative of the arrangement above described, the automatic means for performing a switching operation to reduce the voltage between the electrodes may be actuated by means responsive to the sense of current flow in the two phase lines of the supply. For example, a transformer may have two windings on a comin series with one of the welding electrodes so that the flow of welding current causes magnetisation of the core of the transformer. The two windings of the transformer may be arranged to act as current regulating chokes.

In a further alternative arrangement in which the switching operation is effected in response to the sense of current flow in the two phase lines of the supply. two windings on a common core of a transformer may be connected in the two phase lines respectively and the magnetic flux in the core of the transformer may be arranged to complete the normal welding circuit when the magnetic flux is unbalanced, by providing an armature, which is arranged to be attracted towards the core of the transformer under such conditions, but to move away from the core when the magnetic fluxes due to the two windings connected to the two phase lines balance. A feature of this invention consists in using the regulating chokes provided in the welding circuit as the transformer aforementioned, the two chokes for the two lines to the welding electrodes being wound on a common core having three limbs. the windings being wound on two of the limbs and the armature being arranged in association with the third limb of the transformer.

In a further alternative arrangement in accordance wtih the present invention the automatic means for reducing the voltage between the two electrodes is arranged tobe responsive to the voltage existing between one of the two phase lines and the neutral or common return line or alternatively to be responsive to a comparison of'the voltages between each of the lines and the neutral or common return line. It can be shown that owing to the presence of the current regulating chokes in the two phase lines the voltages between the phase lines and the neutral become unequal when the main welding arcs between the electrodes connected to the phase lines and the work connected to the neutral are broken. This change of voltage or the inequality of the two voltages can be arranged to operate voltage responsive means to reduce the voltage between the electrodes. Where means responsive to the voltage between one line and the neutral is employed it is preferably connected to the line in which the change of voltage-to-neutral is greater, this being dependent upon the sense of phase rotation of the welding current supply.

In the arrangements above described the reduction of the voltage between the electrodes, in response to the change in the current or voltage conditions which occurs when the arcs between the electrodes and the work are extinguished, has been eflectedby means responsive to the current balance or voltage conditions in the welding circuit but, in accordance with the present invention, such control may alternatively be effected in response to the change of conditions in the rimary windings of the welding transformer. For instance, when the two-phase welding supply is derived through a Scott-connected transformer from a three-phase alternating current supply the control may be effected in response to the unbalancing of the primary currents in the three phases of the supply to the primary windings of the welding transformer which occurs when the welding arcs between the electrodes and the work are extinguished leaving only the persistent are between the two electrodes. This of! balance of the primary currents may be detected by means of a core balance transformer or a bridge network or by balanced relays energised by the current flowing in any two of the three-phase alternating current supply lines.

Seven arrangements of apparatus in accordance with the present invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 shows an arrangement in which control is effected in response to' the current in the neutral or common return lead of thewelding supply. Figure 2 shows a modification.

Figure 3 shows an arrangement in which control is effected in response to the phase relationship of the currents in the two phase lines feeding current to the electrodes, and Figure 4 is a vector diagram showing in full lines the phase relationship of the currents fed to the welding electrodes during welding and in dotted lines the phase relationship of the currents fed to the electrodes when the welding arcs between the electrodes and the work are broken.

Figure 5 shows an alternative arrangement in which the currents are fed to the electrodes through two windings disposed on separate limbs of a transformer having three limbs, the third limb carrying a winding by which a contactor is energised during welding.

Figure 6 shows a further alternative arrangement in which the currents are fed to the electrodes through two windings of a double-wound choke having high leakage inductance the windings being tapped so that they may be used for regulating the welding current. The magnetic flux in the third limb of the transformer is arranged to attract an armature to actuate a switch for controlling the supply of welding current to one of the electrodes.

Figure 7 shows another alternative arrangement in which the change of voltage between one of the electrodes and the work as a result of a change from the welding condition in the condition in which only an inter-electrode are persists between the two electrodes is used to control the welding current.

Figure 8 is a vector diagram for the arrangement shown in Flam-e 7 with one direction of phase rotation and Figure 9 is a vector diagram with the opposite phase rotation. In the vector diagrams the full lines indicate the voltage vectors during welding and the dotted lines the vectors when the arcs between the electrodes and the work are broken.

Figure 10 shows afurther alternative arrangement in which a current balance transformer is associated with the primary winding of a Scottconnected welding transformer of which only the primary connections are shown.

Referring to Figure l: a two-phase supply welding current is fed to two leads II and H respectively and a neutral lead it. The leads I. and II are connected to electrodes l3 and It respectively, while the work It which is to be welded is connected to the neutral lead it. The two-phase welding current supply is obtained from the secondary windings l8 and ll of a Scott-connected transformer indicated at II. The transformer it has a primary winding 28, one end of which is connected to one three-phase line 2| while the other end is connected to the centre point of a second primary winding 22, the ends of which are connected to the two other three-phase lines 23 and II respectively. The connection to the line 24 is made through a contactor 25 which is closed when its operating coil 26 is energised.

As above mentioned, an end of each secondary winding 16 and II of the transformer II is connected to the leads l0 and II respectively. The

other ends of the secondary winding I C and I1 urable current transformer 36 having a primary winding 3| arranged to carry the current in the neutral lead I2 and having a secondary winding 32 which feeds through a rectifier 33 the operating winding 34 of a current relay having contacts 35 and 36 which are bridged when the be connected through additional sections 48 and 49 of the current regulating chokes 28 and 23 to the common return lead [3 which is connected through the winding 3I of the saturable current transformer 30 to the lead I2 connected to the I work.

operating winding 34 is energised. The contacts 35 and 36"are connected in a circuit for energising the operating winding 26 of the line contactor from the lines 2I and 24. winding 40 of a voltage relay is connected in series with a rectifier 4I between the lead I9- and the lead II. The voltage relay has contacts 42 and 43 which are connected in parallel with the contacts 35 and 35 of the current relay.

The arrangement is such that while welding current flows in the neutral lead I2 the line contactor connecting the line 24 through the contacts 25 to the primary winding 22 of the transformer I8 is maintained closed but when the current in the neutral lead I2 ceases when the welding electrodes are removed from the work, the cessation of the current through the primary winding of the saturable current transformer de-energises the operating winding 34 of the current relay and so de-energises the operating winding 26 of the line contactor thereby cutting oil the supply from the line 24 to the primary winding 22 of the transformer III.

The function of the voltage relay is to respond to a drop in the voltage between the leads I I and I9 such as occurs when the electrode I4 is touched on the work l in order to re-establish welding conditions. The reduction in voltage partially deenergises the operating winding 40 allowing the contacts 42 and 43 to close thereby energising the operating winding 26 of the line contactor so as to complete the three-phase connections to the primary windings of the transformer [8. The completion of these connections to the primary windings of the transformer re-establishes the supply of welding current to the leads I 0 and II and the neutral lead I2 so that current again flows in the primary winding 3| of the saturable current transformer which results in energisation of the operating winding 34 of the current relay causing contacts 35 and 36 to close so as to maintain the energisation of the operating winding 26 of the line contactor even when the voltage between the leads II and I9 rises during welding to a value which causes the contacts 42 and 43 to separate.

An alternative arrangement which in many respects is similar to that already described is shown in Figure 2 of the accompanying drawings. The same reference numerals will be used to indicate corresponding parts in the two arrangements. In the arrangement shown in Figure 2, the secondary windings of the Scott-connected transformer are provided with tappings so as to enable the welding voltage to be altered and a changeover switch generally indicated at 31 is provided for this purpose. When the switch 31 is in the left-hand position only the portions I6 and ll of the secondary windings are used and these are connected together and through the lead I9 and the winding 3| of the saturable current transformer 30 to the lead I2 connected to the work. When the changeover switch 31 is in its right-hand position, additional windings 38 and 39 which constitute continuations of the secondary windings I6 and I1 respectively are brought into circuit. These are arranged to The operating As in the arrangement shown in Figure 1, the operating winding 34 of the current responsive relay having normally open contacts 35 and 36 is arranged to be fed from the secondary winding 32 of the saturable current transformer 30. The rectifier 33 in this circuit is shown as being a bridge connected rectifier. The connections of the voltage responsive relay having the operating winding 40 are similar to those shown in Figure 1 except that this relay is connected between the lead connected to the welding electrode I I3 and the junction of the secondary winding I6 and 38. The rectifier 4| through which the operating coil 40 is fed is of the bridge connected type.

The arrangement of the connections for feeding the operating winding 26 of the contactor having contacts 25 controlling the supply of cur-' rent to the centre tapped winding 22 of the Scottconnected transformer is somewhat different from that shown in Figure 1. The operating winding 26 is fed through a bridge connected rectifier 44, one alternating current terminal of which is connected to the junction of the secondary windings I6 and 36 while the other alternating current terminal of the rectifier 44 can be connected through three alternative pairs of contact to the end of the secondary winding I6 which is connected to the current regulating choke 28. The three alternative pairs of contacts are the normally open contacts 35 and 36 of the current responsive relay, the normally closed contacts 42 and 43 of the voltage responsive relay, and a. further pair of contacts 45 and 46 which can be closed by a manually operable cam 41. When the cam 41 is in the position in which the contacts 45 and 46 are open, the apparatus is arranged for two-phase welding using the control arrangements which operate generally in the manner above described with reference to Figure 1. If, however, the contacts 45 and 46 are closed by the cam 41, the control arrangements are rendered inoperative and either the electrode l3 or the electrode I4 or each of these electrodes can be used for single-phase welding, the work in each case being connected to the neutral or common return lead I2.

Referring to Figures 3 and 4; the electrodes I3 and I4 are fed from two-phase welding current supply lines 5I and 52 through current regulating chokes 53 and 54 and also through a double current transformer 55. The work I5 is connected to the common return or neutral lead I2. A normally open contactor 56 having an actuating coil 51 is arranged to interrupt the supply of current to the electrode I4 when the operating winding 51 is deenergized. The double current transformer 55 has two windings which carry the current fed to the leads I 0 and I I connected to the electrodes I3 and I4 respectively. These windings are magnetically coupled to a third winding 53 connected to the operating winding 59 of a relay 60 which has normally open contacts.

Under welding conditions the vector relationship of the currents fed to the electrodes I3 and. I4 is generally as shown in full lines in Figure 4, the vector 0A representing the current fed to the electrode I3 being generally at right angles to aesaavr 7 the vector OB representing the current fed to the electrode ll. Under these conditions the 'return current to the neutral line it of the twophase welding supply flows from the electrodes when the welding arcs between the electrodes l3 and I4 and the work II are broken, for instance by moving electrodes II and away from the work, only the inter-electrode are between the electrodes II and I4 persists. Under these conditions a single phase current flows from one electrode to the other so that, as shown in dotted lines in Figure 4, the two current vectors C and OD, representing the currents in the leads I0 and .II respectively, are opposed to one another so that no voltage is induced in the winding 58 of the double current transformer 65 so that the .relay 80 is de-energised thereby de-energising the operating winding 51 of the contactor having :the contacts 58 through which welding current is fed to the electrode H. The opening of the contactor breaks the circuit from the welding supply through the electrodes and thereby breaks the :inter-electrode arc. The welding conditions can .be re-established by touching the electrode 13 on the work I! so that current flows in the lead 10 and thereby induces a voltage in the third winding 58 of the double current transformer so as to energise in turn the relay 80 and the con- =tactor having contacts 56 through which current .is fed to the electrode II from the other phase line 52 of the welding current supply.

The operation of the alternative arrangement :shown in Figure is somewhat similar to that shown in Figure 3. Current from one phase 5| of the welding current supply is fed to the electrode it through one winding of a transformer .having a three limbed core 1|. Current is fed :from the other phase line 52 of the welding cur- :rent supply through the contacts 56 of a normally open contactor and a second winding 12 on a separate limb of the core H of the transformer. A third winding I3 on the central limb of the core II is connected to the operating winding 51 of the contactor having the contacts 58.

During welding the vector relationship between the currents in the windings 10 and I2 is generally as indicated by the vectors 0A and CB respectively in Figure 4. This out-of-phase relationship between the currents causes a voltage to be induced in the winding II which is suflicient to maintain the contactor closed so that the electrode i4 is connected to the second phase line 52 of the welding current supply. When the welding arcs between the electrodes l3 and i4 and the work It are broken. a single phase current flows through the windings Ill and I2 and through the leads II and II and electrodes l3 and II. Under these conditions the magnetic fluxes induced in the core H substantially balance so that no voltage is induced in the winding TI, and the contacts 58 or the contactor open and disconnect the electrode is from the welding current supply. The welding conditions can be re-established by touching the electrode I! on the work it so that it rent fed to the electrode II.

a voltage is induced in the winding It to energise the operating winding 51 of the contactor and thereby close the contacts 5' to complete the twophase welding circuits.

5 In the arrangement shown in Figure 6. welding current from the supply line II is fed to the lead II and thence to the electrode it through a winding ll on one end limb of a three limbed core II. The winding ID has a tapping $2 or a plulO rality of tappings so that either the whole or a part of the winding III may be arranged in circuit. In this way the winding 80 can be arranged to operate as a current regulating choke for adjustingthe magnitude of the welding cur- The other electrode I4 is similarly fed through a second winding 83 \onthe 'Bther end limb of the core ll, this second winding 83 also being provided with a tannin ll or tappings for the purpose above mentioned.

An armature 86 is pivotally mounted in association with a gap between the central limb and the yoke of the core Bl so that when magnetic flux flows through the central'limb it the armature 86 is attracted towards the core. Such movement of the armature It is resisted by a spring 81, the other end of which is connected to a fixed or adjustable support (not shown). The armature I8 is mechanically coupled to switch .contacts 88 which are arranged in the circuit from the supply line 52 to the electrode II. The magnetic flux produced in the central limb of the core 8| is arranged to be greater under the welding conditions than when the inter-electrode are alone exists. The switch contacts 88 are of the normally open type so that they are closed to maintain the supply of welding current to the welding electrode i4 during welding.

The arrangements as above described are applicable. to welding circuits in which a single in welding transformer or generator is arranged to supply welding current to two or more operators simultaneously. In such an arrangement each operator may have individual current regulating chokes and be provided with an indi- 3 vidual transformer having windings carrying the current fed to his two electrodes. Unbalanced currents in the transformer may be arranged to maintain a contactor in one of the two phase lines closed. The operation of this contactor will be independent of the operation of other contactors in the lines supplying other welding operators. This arrangement will ensure that the arc persisting between the two electrodes when they are removed from proximity with the 55 work will be extinguished and automatic reconnection when welding is resumed.

In the arrangement shown in Figure 7, the change in the voltage between one of the welding electrodes and the neutral line as between the 50 welding condition and the condition in which an inter-electrode arc persists between the electrodes l3 and II is arranged to eifect the automatic control of the supply of welding current. In this arrangement the welding electrode I3 is fed from 5 one line SI of the two-phase welding current supply through a current regulating choke It. The other electrode i4 is similarly fed from the other phase line 52 through a current regulating choke 54. The operating winding SI of a voltage re- 7 sponsive relay having contacts ll is connected between the neutral or common return line I! and the lead ll supplying welding current to the electrode H.

The operation of the apparatus shown in Fig ure 7 will be more clearly understood by reference to the Figures 8 and 9 in which the full lines represent the vector relationship or the volt.- ages under welding conditions. In Figure 8, the vector OE represents the voltage between the neutral line I9 and the line 5| while the vector OF represents the voltage between the line I and the line 52. These are the line voltages of the two-phase welding current supply and are assumed to remain substantially constant. During welding the current in the line and the lead I0 to the electrode I3 is represented by the vector OP. The vector OM in phase with the current vector OP represents the voltage between the line I9 and the lead I0 while the voltage across the regulating choke 53 is represented by the vector EM. Similarly, the vector 09, represents the current in the line 52 and the lead II and the vector ON the voltage between the line I9 and the lead II and the vector FN the voltage drop across the regulating choke 54.

When the main welding arcs between the electrodes I3 and I4 and the work I5 are broken the vector relationship is altered and the vector FG represents the single-phase current while the vector EF represents the voltage between the lines 52 and 53. There is a voltage drop in the regulating choke 53 represented by the vector EH and a voltage drop in the regulating choke 54 represented by the vector FJ; The vector HJ represents the inter-electrode voltage. The volt age between the electrode I3 and the neutral or common return line I9 is represented by the vector OH while the voltage between the electrode I4 and the neutral line I9 is represented by the vector JO. It will be seen that the magnitude of the voltage between the neutral line I9 and the electrode I4 represented by the vector H0 is substantially greater in the single-phase condition than the vector ONwhich represents the voltage between these points during welding.

Figure 9 is a vector diagram similar to Figure 8 except that the direction of phase rotation has been reversed. During normal welding conditions the current in line 5I and lead I0 is represented by the vector 0Q and the voltage between the line I9 and the lead II by the vector OR, the voltage drop across the regulating choke 53 being represented by the vector ER. The current in the line 52 and lead 5| is represented by the vector OP, the voltage between the line I9 and the lead II being represented by the vector OS and the voltage drop across the regulating choke 54 by the vector FS. When the main welding arcs between the electrodes I3 and I4 respectively and the work I5 are broken, the vector relationship changes to that shown in dotted lines. The vector FT represents the singlephase current and the vectors EK and FL the voltage drops across the regulating chokes 53 and 54 respectively while the vector KL represents the inter-electrode voltage. The voltage between the neutral line I9 and the line I0 is represented by the vector KO while the voltage between the line I9 and the lead I I is represented by the vector 0L. It will be seen that the vector 0L is substantially greater than the vector OS which represents the voltage between the same points during normal welding.

With the sense of phase rotation shown in Figure 8 the operating winding 90 of the voltage sensitive relay having the normally closed contacts 9| would be connected between the neutral or common return line I9 and the lead l0 because there is a greater change of voltage between the welding condition and the condition in which only the inter-electrode arc exists. With the sense of phase rotation shown in Figure 9, however, the operating winding of the voltage sensitive relay should be connected between the neutral or common retum'line I9 and the lead I I as shown in Figure 7. During welding the-voltage applied to the operating winding 90 of thefvoltage sensitive relay is insunicient to energise the relay so that the normally closed contacts 9| remain closed. On removing the electrodes I3 and I4 so that the welding arcs between them and the work I5 are broken the voltage across the operating winding 90 increases thus causing the contacts 0| to open. The opening of these contacts is arranged to effect a switching operation to reduce the inter-electrode voltage to a value insufilcient to maintain the interelectrode arc. Preferably, where the welding current supply is obtained from a Scott-connected transformer the opening of thecontacts 9| is arranged to open circuit one primary line of the transformer so as to extinguish the interelectrode arcbut leaving a voltage across the operating winding 90 of the voltage sensitive relay greater than the voltage existing across it during normal welding. On re-striking the welding arcs by bringing the electrode to which the operating winding 90 is connected into contact with the work the voltage across the operating winding 90 is reduced to zero or nearly to zero allowing the contacts 9| to close thereby restoring the three-phase transformer connections. The voltage which exists during normal welding across the operating winding 90 is insuflicient to cause the contacts 9| to open.

In the arrangement shown in Figure 10, the control means,, instead of being responsive to the current or voltage obtaining in the welding circuit which may, for example, be the circuit connected to the secondary windings of a welding transformer, the central means are arranged to respond to the unbalancing of the currents in the primary winding of a welding transformer which may, for instance, be of the Scott-connected type or some other type having balanced primary currents from the three phases of a three-phase alternating current supply. In the arrangement shown in Figure 10, a Scott-connected transformer is used, the two-phase welding current supply fed to the two phase lines 5I and 52 and the neutral or common return line I9 being obtained from the secondary windings I00 and IM of a Scott-connected transformer indicated generally at I02. The centre tapped primary winding I03 of the transformer I02 is connected at one end to a three-phase supply line. I04 while the other end of the primary winding I03 is connected through the contacts I05 of a normally open contactor to a second line I06 of the three-phase alternating current supply. The other primary winding ml of the transformer I02 is connected at one end to the centre point of the winding I03 and is fed with current from the third line I08 of the three-phase alterhating current supply through the operating coil I09 of a relay having contacts IIO which are normally open. A core balance transformer III is arranged to produce a voltage in its secondary winding II2 when substantial unbalance exists between the three-phase lines feeding the pri mary windings I03 and I0! of the transformer I02. The secondary winding H2 is connected to the operating winding II3 of a relay having normally closed switch contacts I I4. The switch contacts I I4 are connected in series with the cons,oss,277

tacts N of the relay responsive to the current in the line I00 and when both sets of contacts are closed ,the operating winding III of the contactor having contacts I00 in the three-phase line I00 is energised. In this condition the operating winding II! is connected between the three-phase lines I00 and I".

During welding, the currents in the threephase lines I04, I06 and I00 substantially balance so that no voltage is induced in the secondary winding III of the current balance transformer Iii. The operating winding ill is, therefore, not energised so that the circuit through the switch contacts H0 and Ill is completed to energise the operating winding I II of the contactor to keep the contacts I closed. If the main welding arcs between the electrodes and the work are broken, the currents in the three-phase lines I00, I00 and I08 no longer balance so that a voltage is induced in the winding H2 which energises the operating winding Ill and opens the contacts III thereby de-energising the operating winding II5 to open the contacts I05 through which one end of the primary winding I03 is fed from the line I08 of the three-phase supply. The disconnection of this end of the primary winding I03 causes a reduction in the interelectrode voltage sumcient to extinguish the inter-electrode'arc. The flow of current in the operating winding I09 is notsumcient in this condition to keep the contacts IIO closed.

When it is desired to re-establish the welding condition one of the electrodes connected to the welding supply lines Si or 02 is touched on the work connected to the line I9 so that current is drawn from the transformer thereby raising the single-phase current drawn between the lines I04 and I 00 of the three-phase current supply above the low value which exists due to magnetising current when the arcs between the electrodes and the arcs between the electrodes and the work are extinguished. This increase in current through the operating winding I09 causes the contacts 0 (which had opened under the no load condition) to close. As equal and opposite currents flow in the lines [0| and I00 of the three-phase supply there is no unbalance in the current balance transformer IIO so that the contacts I it are closed. Thus, the operating winding I I5 of the contactor is energised causing the contacts I05 to close so that current is fed through the third phase line I00 to the primary winding I0l of the transformer I02. short transition period the currents in the current balance transformer III return to their balanced state so that the contactor contacts III! are maintained closed during the welding operation. Provided the relay having the operating winding H3 is slugged the momentary unbalance in the core balanced transformer during the transition from the single-phase to the threephase condition will not cause the contactor to open.

I claim:

1. Two-phase welding apparatus with which two electrodes are arranged tobe used simultaneously and in close proximity. said electrodes and the work being arranged to be connected to two phases and the common return lead respectively of a two-phase welding current supply, comprising a Scott-connected transformer for providing said two-phase welding current supply from a three-phase supply, and means responsive to the current flowing in the common return lead to reduce the voltage betwe n .12 electrodes to a value at which the intereiectrode are is extinguished by disconnecting one of the three-phase lines from the Scott-connected transformer.

2. Two-phase welding apparatus according to claim 1. wherein the means responsive to the current flowing in the common return lead includes a saturable current transformer.

3. Two-phase welding apparatus according to claim 1. wherein the means responsive to the current flowing in the common return lead comprises a' current relay, and a contactor in one of the three-phase lines connected to one end of the center tapped winding of the Scott-connected transformer arranged to be controlled by said current relay.

4. Two-phase welding apparatus according to' claim 1, having a voltage responsive relay connected between the common return lead of the welding current supply and one of the two-phase welding current supply leads, and a contactor in one of the three-phase lines to the Scott-connected transformer arranged to be operated by 7 phase welding current supply from a three-phase supply connected to the primary windings of said transformer means responsive to the current flowing in the common return lead including a saturable current transformer, having a primary winding connected in said common relead and having a secondary winding, a

After a.

current relay connected to said secondary winding, a contactor in one of the three-phase lines connected to one end of the center tapped primary winding of the Scott-connected transformer arranged to be energized through contacts of said current relay and arranged to be opened when the flow of current in said common return lead ceases, and a voltage responsive relay connected between the common retum lead of the welding current supply and one of the two-phase welding current supply leads and having contacts arranged to eiiect the closing of said contactor when the voltage between said leads drops below a predetermined value.

6. Two-phase welding apparatus with which two electrodes are arranged to be used simultaneously and in close proximity, said electrodes and the work being arranged to be connected to two phases and the common return lead respectively of a two-phase welding current supply, comprising a Scott-connected transformer having primary windings for connection to a threephase supply and secondary windings for providing said two-phase welding current supply, said secondary windings having tappings for the purpose of voltage adiustment, and means responsive to the current flowing in the common return lead to reduce the voltage between the electrodes to a value at which the inter-electrode are is extinguished by disconnecting one of the three-phase lines of the Scott-connected transormer.

'7. Two-phase welding apparatus with which aesaavv tively of a two-phase welding current supply.

comprising a Scott-connected transformer having primary windings for connection to a threephase supply and secondary windings for providing said two-phase welding current supply, said secondary windings having tappings for the purpose of voltage adjustment, at least one current regulating choke in the welding current supply having a portion arranged to be switched into and out of the circuit when the secondary voltage is changed by alteration of the tapping, and means responsive to the current flowing in the common return lead to reduce the voltage between the electrodes to a value at which the inter-electrode arc is extinguished by disconnecting one of the three-phase lines of the Scottconnected transformer.

8. Two-phase welding apparatus according to claim 7," wherein the means responsive to the current flowing in the common return lead comprises a current relay, and a contactor in one of the three-phase lines connected to one end of the center tapped primary winding of the Scott-connected transformer arranged to be controlled by said current relay.

9. Two-phase welding apparatus according to claim 8, having a voltage responsive relay connected between the common return lead of the welding current supply and one of the two-phase welding current supply leads and having contacts arranged to effect the closing of said contactor when the voltage between said leads drops below a predetermined value.

10. Two-phase welding apparatus according to claim 9, having means for rendering the contactor operating means ineilective when it is desired to perform single-phase welding with the apparatus.

IAN JACK PICKERING.

References Cited in the ille of this patent UNITED STATES PATENTS Name Date Holslag Mar. 8, 1921 Bethenod ..-Dec. 14, 1926 Number 

